XIII 



PHYLUM CHORDATA 



141 



of the cesophageo-cutaneous duct, and, in Myxine (Fig. 807, br. &.), 

 one on the right side supporting 

 the common external gill -tube. 



The myotomes of one side al- 

 ternate with those of the other. 



The intestine is very wide. 

 The liver consists of two separate 

 portions, the ducts of which open 

 separately into the gall-bladder. 

 A pancreas-like gland is present 

 in both Myxine and Bdellostoma. 

 The brain differs considerably 

 from that of the Lamprey, espe- 

 cially in the larger hemispheres, absence of lateral ventricles, 



. 80S. Auditory organ of Myxine. 

 amp., amp. 1 ampulla? ; cmL s. endolym- 

 phatic sac ; s. c. semicircular canal ; utr. 

 sac. utriculo-saeculus. (After Retzius.) 



and smaller mid-brain. 



O 



efferent artery. 



C " lpai ' ati< 



The dorsal and ventral roots of the 

 spinal nerves unite instead of remaining 

 separate. The eyes are vestigial and 

 sunk beneath the skin, and the auditory 

 organ (Fig 808) has only a single semi- 

 circular canal, which, having an ampulla 

 at each end, probably represents both 

 anterior and posterior canals. 



Bdellostoma has a persistent prone- 

 phros in the form of a paired irregularly 

 ovoidal body situated just above the heart 

 and consisting of a large number of 

 tubules richly branched peripherally : the 

 nephrostomes open into the pericardium. 

 The tubules do not communicate with 

 the pronephric duct in Myxine, but end 

 blindly : in Bdellostoma they open into 

 an incomplete longitudinal duct, which 

 does not communicate with the per- 

 manent kidney-duct. The functional 

 kidney is the mesonephros, and is specially 

 interesting from the fact that in Myxinoids 

 it retains in the adult its primitive seg- 

 mental arrangement. The ureter (prone- 

 phric duct, Fig. 809, a) sends off in each 

 segment a coiled tubule (b) with a single 

 Malpighian capsule (c), into which a 

 branch from the aorta (d) enters and 

 forms a glomerulus. 



Myxine is hermaphrodite, the anterior 

 part of the gonad being ovary, the pos- 

 terior testis : in some the ovary is mature 

 and the testis rudimentary, in others the 



